Herbert L. Hawkins — Studies on the Echinoidea. 497 



the correspondiDg plates of an Echinarachnms of 6'5nun. ditimeter 

 (about twice that of Pygastrides), figured by Loven (Etudes, 

 pi. i, fig. 245), are practically identical in proportions with those of 

 Echi7iocyamus. In the Clypeastroids, however, the succeeding 

 ambulacral plates are high and hexagonal, unlike the " Cidaroid " 

 arabulacrals of Pygastrides. Since all coronal plates undergo 

 considerable changes in size, and often in shape, during the growth 

 of the test, this difficulty, though real, is not insurmountable. The 

 " Bothriocidaroid " character of the extra-petaloid ambulacrals of 

 the Clypeastroids is a sign of their obsolescence — they would hardly 

 be in that condition when first developed. 



The globular form of Pygastrides makes it difficult to suppose that 

 it would develop into a discoid Scutellid, although such a change 

 would be by no means impossible. However, most of the Fibulariidae, 

 especially Fibularia itself, have elevated tests that do not differ 

 seriously in their proportions from those of Pygastrides. 



The peculiar raultiporous plates of ttae ambulacra of the 

 Fibulariidae are certainly very different from those of Pygastrides, 

 but there are indications of disturbance in the latter. The 

 anomalous nature of the pore-pairs, with a round internal pore, and 

 their position near the adapical margins of the plates, seem to 

 indicate some aberrant development. The rows of pores on the 

 Fibulariid ambulacrals are similarly placed. 



A review of the above arguments gives the following conclusions. 

 The position of the sphaeridia, if it be a reliable character, points 

 definitely to the Fibulariid, Laganid, or Scutellid nature of the adult 

 of Pygastrides. The relations of the proximal coronal plates do not 

 carry the argument much further, but they are generally Clypeastroid 

 in character, with some resemblances to the Cassiduloid quality. 

 The general shape of the corona is more suggestive of a Fibulariid 

 than of any other likely adult. So that the balance of evidence 

 tends to indicate that Pygastrides is an early post-larval stage of 

 some such genus as Fibularia. The characters of the perignathic 

 girdle are not antagonistic to such a conclusion (see section 7), but 

 the nature of the ambulacra introduces a difficulty. In JEchi7iocyamus 

 and its allies, the extra-petaloid ambulacrals are high, and each 

 plate is perforated by numerous minute pores arranged, for the most 

 part, transversely near the adapical suture. This extremely 

 specialized, perhaps degenerate, condition must have been derived 

 phylogenetically from a more normal biporous state, and there is 

 every reason to expect that such a change would be repeated in 

 ontogeny. The features in which the podial pores of Pygastrides are 

 aberrant all point towards a Fibulariid modification more than to any 

 other. 



I therefore incline strongly to the opinion that Pygastrides relictus 

 is an early phase in the post-larval ontogeny of some species of the 

 Fibulariidae. The conclusion reached at the close of the previous 

 section — that Pygastrides is a young stage of some form less remote 

 from the primitive Holectypoid condition than EcMnon'eus — is 

 thus concordant with the result of this independent line of 

 argument. 



DECADE VI. — VOL. V. — NO. XI. 32 



